Embodiments of the invention relate generally to systems and methods for driving light emitting diodes.
A light emitting diode (LED) is a photoelectric conversion device, which is operable to emit light in response to electrical current or voltage supplied thereto. Generally, the LED has an N-type semiconductor and a P-type semiconductor joined together. The LED emits light through recombination of electrons and holes. Such an LED is widely used for signaling, traffic light, backlighting, and general illumination due to its high efficacy, energy saving, environmental friendliness and long lifetime.
When the LED is directly connected to an AC voltage source, the LED may not continuously emit light in a full cycle. To solve this problem, a LED that can be used while being connected directly to an AC voltage source has been disclosed in PCT patent application publication No. WO2004/023568A1, entitled “Light-emitting device having light-emitting elements” by Sakai et al. According to the disclosure of Sakai et al., two LED arrays are connected to each other in reverse parallel. One LED array operates in a first half cycle (or positive half cycle) of an AC voltage source, and the other LED array operates in a second half cycle (or negative half cycle) of the AC voltage source.
As disclosed by Sakai et al., the two LED arrays alternately cycle on and off in response to a phase change of an AC voltage source. This results in some operating issues for the LED. The first is that when the AC voltage from the AC voltage source fluctuates, the current flowing through the LED changes accordingly. Thus, stable and constant brightness of the LED may not be obtained. The second is a poor power factor and total harmonic distortion (THD) because the LED begins to emit light only when the AC voltage exceeds a threshold voltage. The third is that it's difficult to perform dimming control of the LED in some applications. The fourth relates to the flicker phenomenon, which although not observable with the naked eye, will cause eye fatigue if the LEDS are used for illumination for a long period of time.
It is desirable to provide system and method of driving the light emitting diodes to address the above-mentioned problems.